ASoC: wm_adsp: Remove private field from wm_coeff_ctl

The private field in wm_coeff_ctl is currently unused and given the
controls are entirely handled within the ADSP code it is not clear what
it would be used for in the future. Remove the field for now it can be
readded if it is ever required.

Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@kernel.org>